The prior art in this field embraces a process in which metal strips of suitable gage and width are cut and shaped into a round profile, usually 3/8" across, by repeated passes through a forming machine to the end of obtaining a composite tube the edges of which are welded together directly in the machine; the edges of the strips are first united by pressure from forming rolls in order to attenuate thermal expansion during the welding step, for which a plasma-arc torch is employed, utilizing an argon gas shield (no use is made of solder, as the fused tube must retain uniform shape during subsequent swaging operations, and any molecular break-up that would be caused by the introduction of a more ductile alloy has therefore to be avoided). Needless to say, the initial gage of such metal strip must be heavy enough to ensure that the plasma-arc process remains efficient. The continuous composite tube obtained in this way is then suitably annealed, whereupon its annular cross section is reduced by rotary swaging, which involves slipping the formed tube over a mandrel taking the form of a rod, one end of which is anchored, the other integral with a hardened pin of larger dimensions designed to bear the impact of the contoured swages; the swages rotate against the outer surface of the tube in such a way as to bring about a gradual reduction in the annular cross section, while the tube itself, gripped in the clamp of a chain-drive drawing-bench, is extended by a carriage hitched to the chain via a spike.
This initial reduction of the tube's annular cross section is followed by an intermediate annealing stage, whereupon the swages and the mandrel are changed, and a further reduction of the annular cross section is brought about; further annealing the swaging passes are implemented in this way until a tube of approximately 5/32" diameter is produced.
A continuous wire is now threaded into the tube to serve as a solid core, and the tube subjected to repeated drawing until brought down to the final diameter envisaged, say 3/32". In such an instance, the composite drawn tube ultimately produced will exhibit an outer annular section of some 0.0070" thickness. Clearly enough, the selection of a greater or lesser final thickness will dictate the gage of strip to be adopted when forming the tube initially, though it will be remembered that a certain minimum gage is essential if the plasma-arc weld is to give efficient results.
Where the requirement is for a hollow member, the tube is transferred to a further machine designed for removal of the core wire (copper or steel). This removal operation becomes necessary where ultimate thickness of the tube wall needs to be of the order of between 0.0039" and 0.0059". The machine utilized consists of a steel beam with tracks, some 20 ft long; one end of the tube is clamped between grips, mounted to a carriage that traverses the entire length of the beam, and the remaining end made fast between further grips integral with a hydraulic cylinder. With both ends held in this manner, a measure of tensile stress is imparted to the tube; pincers with cutting edges are used to sever the wall of the tube at either end, and the cylinder, actuated by a hydraulic power pack, begins pulling on the core wire. The diameter of the core wire reduces under tensile stress, and can be easily removed from the surrounding tube, which thus remains free.
Manufacturing methods of the prior art type thus outlined are not able however, to produce hollow members which exhibit a variegated appearance and are embellished with localized visual features composed of different colors.
Accordingly, it is an object of the invention to create the possibility of embodying variegated hollow members with color-enhanced geometry and ornamental features, in an infinite variety of ways.
It is likewise an object of the invention to provide for the embodiment of hollow members in such a way that a solid core may be retained, in order to permit utilizing the members for forming chain or necklace links, and removed following the formation of such links.
A further, important object of the invention is to provide for the embodiment of a hollow member, in cases where a decidedly thin annular cross section is required, by way of seam-folding together a number of metal strips or moldings arranged edge to edge and gathered into a tube, in such a way that their subsequent fusion by plasma-arc methods need not be conditioned by the initial gage of the metals employed.